Update python-altair to version 4.2.0 / rev 11 via SR 979235
    
    https://build.opensuse.org/request/show/979235
    by user mcepl + dimstar_suse
    - Upgrade to 4.2.0:
      - Pie charts are now supported through the use of mark_arc.
      - Support for the datum encoding specifications from Vega-Lite;
        see Vega-Lite Datum Definition.
      - angle encoding can now be used to control point styles
      - Support for serialising pandas nullable data types for float data
      - Automatically create an empty data object when Chart is
        called without a data parameter
      - Allow the use of pathlib Paths when saving charts
      - Support deepcopy for charts
      - Fix to_dict() for nested selections
      - Fix item access for expressions